The present invention relates generally to windings for electric induction apparatus and particularly to winding arrangements for disc-wound interlaced windings that will significantly reduce the axial voltage gradient and therefore the electrical insulation required between particular conductor turns at the inner surface of such windings, where the combination of axial voltage gradient and radial voltage gradient to an adjacent inner winding or ground, causes an increased resultant voltage gradient.
It is well known that highly inductive windings such as iron core transformer and reactor windings, when exposed to steep wave front impulse or transient voltages, initially exhibit an exponential distribution of voltage drop along the length of the winding with a very high voltage gradient appearing at the first few turns adjacent the high voltage end. This condition arises because such windings present a predominantly capacitive impedance to steep impulse voltages. Such capacitive impedance is made up of a complex network of capacitances in a series and parallel relation. If series capacitance only were present, voltage distribution throughout the winding would be substantially uniform and linear. The larger the series capacitance with respect to the parallel or ground capacitance of such windings, the more uniform the initial distribution of steep wave transient or impulse voltages applied to such windings.
One common type of high voltage winding for transformers and reactors is the so-called disc winding wherein each of a plurality of annular coils is wound on a core as a radial spiral, the coils being disposed co-axially on the core and connected electrically in a series circuit relation. In such a disc winding it is known that series capacitance may be increased and impulse voltage distribution improved by constructing the winding in groups of coils called winding sections, with each winding section having at least two coils, the coils of a winding section being connected together in a re-entrant series interlaced relation, as illustrated in U.S. Pat. No. 2,453,552 to STEARN.
Cross-connections between the disc coils of a winding section usually occur in well-defined cross-connection regions that are generally aligned with each other and with the longitudinal axis of a winding. These cross-connection regions extend a limited distance circumferentially of the winding. Due to the nature of an interlaced winding, a conductor at the internal surface of such a winding providing the cross-connections between the disc-wound coils of a winding section is separated from a corresponding internal conductor in an adjacent winding section by only the normal spacing between disc-wound coils. The just-mentioned cross-connection conductor in one winding section and the corresponding internal conductor in an adjacent winding section are connected together in the winding through several conductor turns which creates a substantial voltage gradient between these internal conductors in adjacent cross-connection regions. This voltage gradient and therefore the electrical stress between these conductors is normally compensated for by either increasing the insulation of the affected conductors in the cross-connection region or by increasing the axial separation of the affected disc coils. However, these insulation increasing modifications necessarily result in an increase in the physical dimensions of the winding with attendant weight and cost penalties. In addition, the just-mentioned insulation increasing modifications result in a winding having an irregular surface, requiring greater physical separation between windings in a multiple winding inductive device or between a winding and the core of an inductive device.
Furthermore, a winding design that aligns all of the cross-connections of a winding section with each other and with the longitudinal axis of the winding produces a winding design having less than desirable locations for winding taps and connections.
Additionally, in prior art windings at the interlaced disc-wound type having cross-connections aligned, the mechanical process of bending the conductors from one coil to another and adding some small insulation for mechanical protection, results in a "bulge" or overbuild in the radial dimension of the coil in the cross connection region.
Accordingly, it is the principal object of the present invention to provide an interlaced disc winding that will avoid the necessity of providing additional insulation between particular conductors at the inner surface of such a winding.
Another object of the present invention is to provide an interlaced disc winding that will have physical dimensions of minimal size with attendant weight and cost savings.
Still another object of the present invention is to provide an interlaced disc winding that will have a regular exterior surface.
A further object of the present invention is to provide an interlaced disc winding that will have desirable locations on the winding for taps and connections thereto.